LU-2505 osc: use GFP_ATOMIC for ll_dirty_page_discard_warn()

LU-2505 osc: use GFP_ATOMIC for ll_dirty_page_discard_warn()

Because it can be called inside client obd list lock as follows:

        ll_dirty_page_discard_warn at ffffffffa0a3d252 [lustre]
        vvp_page_completion_common at ffffffffa0a7adfc [lustre]
        vvp_page_completion_write_common at ffffffffa0a7ae6b [lustre]
        vvp_page_completion_write at ffffffffa0a7b83e [lustre]
        cl_page_completion at ffffffffa05eed8f [obdclass]
        osc_completion at ffffffffa0880812 [osc]
        osc_ap_completion at ffffffffa086a544 [osc]
        brw_interpret at ffffffffa0876d69 [osc]

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: I04a4cc1142e019d39dccf2c1f3084c0298acff9d
Reviewed-on: http://review.whamcloud.com/4870
Tested-by: Hudson
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>